Protective devices are widely used in parallel with electronic information processing circuits to protect them against spurious voltage surges and voltage transients. Capacitors are normally used to protect such circuits against high frequency voltage transients which could cause errors in signal processing or in stored signals. Threshold devices such as Zener diodes are normally used to protect such circuits against high amplitude, low-frequency voltage surges which could otherwise cause permanent physical damage to the circuits. Thus, many circuits require both a capacitor and a Zener diode in parallel for proper protection.
The multilayer ceramic capacitor is becoming increasing popular as a protective device because of its high capacitance and its relatively compact size. Ceramic capacitors are normally made by sandwiching layers of metal paste between layers of a ceramic capacitor composition and then sintering the unit to stabilize both the metal, which thereafter constitutes capacitor electrodes, and the high dielectric ceramic capacitor composition. Alternate metal electrodes are then attached to one terminal electrode, and the remaining electrodes are attached to an opposite terminal electrode such that the metal electrodes extending from the terminal electrodes are interdigitated, and a high total capacitance is presented between the two terminal electrodes.
Zener diodes are devices in which a band-gap material such as silicon is contained between opposite terminal electrodes. When the voltage across the terminal electrodes exceeds a threshold value, the resistance of the device abruptly decreases, thus shunting current from the circuit being protected.
A known device that could be used in place of the Zener diode is the varistor, a device that has generally the same external electrical characteristic as the Zener diode, but which operates on a different principle. As will be explained later, a varistor may be a ceramic device in which current conduction occurs by electron tunneling through grain boundaries when the applied external voltage exceeds some threshold value.
A well-known trend in recent times is the increased microminiaturization of electronic circuits, to the point at which an entire electronic system can be contained on a single silicon chip. While multilayer ceramic capacitors are more compact than other comparable capacitors, the ceramic capacitor and Zener diode in parallel with an electronic circuit are often bulkier than the circuit they are protecting. Since the miniaturization of protective devices has not kept pace with the miniaturization of electronic circuits, there is clearly a well-recognized need for further miniaturization of protective devices.